umtx_op: reduce redundancy required for compat32

All of the compat32 variants are substantially the same, save for
copyin/copyout (mostly). Apply the same kind of technique used with kevent
here by having the syscall routines supply a umtx_copyops describing the
operations needed.

umtx_copyops carries the bare minimum needed- size of timespec and
_umtx_time are used for determining if copyout is needed in the sem2_wait
case.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27222